A bridge film between 2 and 3 is certainly possible, and there are numerous stories which could focus primarily on pre-clone wars characters (So You could avoid having to introduce Asoka and Ventriss). Its really unfortunate that a Grevious Origin story was never explored, as he would be a good character to introduce here.

In terms of tone, Episode 2.5 should effectively be 'The Seperatists Strike Back' as far as they should have the Republic on the backfoot, striking numerous victories etc, making the battle of coruscant seem like the crunch point. Grevious should be terrifying as he is in the Gennedy Tartakovsky series.

I'd recommend using episodes from S02E10 onwards as they have the updated animation quality, and are generally better, although they focus less on the war. The use of dubbing/switched dialogue could be effective too.